MacBook built-in line-in is glitchy/buzzy with more CPU use

I'm using Max 5.1.8 on a white MacBook, the late 2006 model.
I'm trying to sample sounds in through the built-in input (line-in) and it's no problem until my my patch's CPU use gets into like 25-30%, whereupon all sounds through that input go glitchy, with a constant buzz. The sampled waveforms look like a pulse-train is being added to the recording somewhere. Sampling method is simply adc~ going into record~. Output sounds remain unaffected at even higher levels of CPU use. The patch works totally fine with a couple of other sound cards. (NB I have a gig with very limited setup time so I'd like to be able to use the built-in input and outputs instead of external sound cards, even though they work.)
The problem I'm having is repeatable on two of my friends' macbooks too, running the same patch.

Has anybody else had this problem? Has anyone managed to get rid of the buzz?
Many thanks
Nick Zh